1. Field of the Invention
The present invention relates to a semiconductor pressure sensor and a method of producing the same. More specifically, the present invention relates to a semiconductor pressure sensor which may be installed in a vehicle or the like in order to measure pressure such as atmospheric pressure, and also relates to a method of producing such a semiconductor pressure sensor.
2. Description of the Related Art
FIGS. 37 and 38 show a conventional semiconductor pressure sensor, FIG. 37 being a plan view, and FIG. 38 being a side sectional view taken along the line B--B shown in FIG. 37. Referring to these Figures, a silicon substrate 11 comprises a silicon substrate having the crystal orientation (1 0 0) is for the purposes of increasing the sensitivity of the sensor with which it measures pressure and including a diaphragm 7 formed by anisotropically etching one surface (reverse surface) of the substrate 11. Gauge resistors 1 are formed at certain positions on the other surface (front surface) which are in the vicinity of edge portions 3 of the diaphragm 7 where the diaphragm 7 deforms most greatly when it is subjected to a change in pressure, the position of each gauge resistor 1 thus corresponding to one of the sides defined by the diaphragm edges. The gauge resistors 1 are interconnected by diffused wirings 2 into a bridge circuit. When the diaphragm 7 deforms due to a change in pressure, the deformation is converted by the bridge-connected gauge resistors 1 into an electrical signal.
Each of the diffused wirings 2 is connected to a metal electrode 4, which is in turn connected to an external circuit. A silicon oxide film 9 is disposed on the front surface of the silicon substrate 11, and serves as a mask during the formation of the gauge resistors 1, the diffused wirings 2, etc., as well as a protective film of the sensor. A glass coating 10 for protecting the whole of the front surface is disposed on the silicon oxide film 9. The diaphragm 7 is formed by, after the device on the front surface has been fabricated, etching the substrate 11 from the reverse surface in correspondence with the positions of the gauge resistors 1. The diaphragm 7 is the portion of the sensor which responds to pressure by deflecting.
The conventional semiconductor pressure sensor having the above-described construction is produced as shown in FIGS. 39 through 42, which are side sectional views of the sensor being produced. First, as shown in FIG. 39, a silicon substrate 11 composed of a single-crystal having the crystal orientation (1 0 0) is prepared. Subsequently, a device is fabricated on the front surface of the silicon substrate 11. Specifically, a silicon oxide film 9 is formed on the relevant surface of the substrate 11 by thermal oxidation. After forming openings in the silicon oxide film 9 at positions corresponding to the positions at which diffused wirings 2 are to be formed, the silicon oxide film 9 is used as a mask, through which impurities, such as boron, are diffused, thereby forming the diffused wirings 2 having a low resistivity. Further, four gauge resistors 1 are formed. The gauge resistors 1 are bridge-connected by diffused wirings 2 (FIG. 40).
Subsequently, certain portions of the diffused wirings 2 are exposed, and metal electrodes 4 are formed on these portions. Thereafter, the device-fabricated surface of the semiconductor pressure sensor is substantially entirely covered with a glass coating 10, such as a silicon nitride film (FIG. 41). Subsequently, an etching mask 13, such as a silicon nitride film, is formed on a part of the reverse surface of the silicon substrate 11 (FIG. 42) in such a manner that the position of the gauge resistors 1 on the front surface will correspond to that region of a diaphragm 7 (formed later) at which the diaphragm 7 is able to deform most greatly when subjected to a change in pressure. Subsequently, an anisotropic etching employing an etchant (such as an alkali-type etchant) is effected from the reverse surface of the silicon substrate 11 until the required thickness of the diaphragm 7 is achieved.
The region at which the diaphragm 7 deforms most greatly when subjected to a change in pressure comprises the edge portions 3 of the diaphragm 7 shown in FIG. 38. Hence, the gauge resistors 1 on the front surface of the semiconductor pressure sensor are positioned above these edge portions 3.
The conventional semiconductor pressure sensor has the following drawbacks: because there is a risk that the thickness of the silicon substrate 11 (silicon wafer) will vary between various portions thereof, and a risk that the etching rate will vary between a plurality of batches, between sensors within one batch, and/or between various portions of the wafer surface, and because the etching is effected to a relatively great depth, it is difficult to control the thickness of the diaphragm 7 with a high level of precision, and the operational efficiency is poor; since the diaphragm 7 is formed in the final stage of the sensor production, it is necessary, at this time, to completely protect the wafer surface; and due to the fact that anisotropic etching has a certain angle at which the etching proceeds and that the extent to which the diaphragm 7 can be made thin is inevitably limited, it is difficult to make the sensor small.